Rectifiers of various designs may be used for feeding direct current systems out of three-phase current systems. The present patent application relates to active, i.e., controlled, bridge rectifiers which have active switching elements, for example in the form of known metal oxide semiconductor field effect transistors (MOSFETs). Bridge rectifiers having a six-pulse design are frequently used in vehicle electrical systems, corresponding to the three-phase current generators which are usually installed there. However, the present invention is similarly suitable for bridge rectifiers having other numbers of phases, for example five-phase generators, and in other use scenarios.
As explained in DE 10 2009 046 955 A1, for example, the use of active bridge rectifiers in motor vehicles is desirable, among other reasons, due to the fact that they have lower power losses compared to passive, i.e., uncontrolled, bridge rectifiers.
However, load shedding is a critical fault in particular in active bridge rectifiers. Load shedding (dumping) occurs when, for a highly excited generator and a correspondingly high delivered current, the load on the generator suddenly decreases, for example, by disconnecting consumers, and this is not intercepted by capacitively acting elements in the direct voltage network (for example, the battery in the vehicle electrical system).
In this regard, in the extreme case the generator may continue to deliver additional energy to the vehicle electrical system for a duration of up to approximately 300 ms to 500 ms. This energy must be converted (cleared) in the bridge rectifier in order to protect downstream electrical components from damage from overvoltage. In passive or uncontrolled bridge rectifiers, this protection is generally provided by the rectifier diodes themselves, since the lost energy may be converted into heat in the rectifier diodes. In presently available active switching elements such as MOSFETs, however, it is not possible to completely simulate these properties. Therefore, additional protective strategies are necessary.
During load shedding, some or all switching elements of the upper or lower rectifier branch may be completely or intermittently short-circuited, as also discussed in cited DE 10 2009 046 955 A1. An appropriate control signal may also be clocked in such a way that the voltage does not fall below a minimum voltage level and does not exceed a maximum voltage level.
However, multiple clocking of the control signal within a half-wave has disadvantages, since this requires an evaluation circuit for each phase, and an appropriate DC link capacitance must be provided for maintaining the vehicle electrical system voltage. This is difficult to achieve due to the thermal constraints and the required vibration resistance at the mounting location on the generator. In particular, however, when corresponding phase short circuits are deactivated, rapid current changes may also occur, which, in combination with the line inductances which are present, result in voltage peaks. As a result, connected components may be damaged.
Therefore, there continues to be a need for improved protective strategies for active bridge rectifiers during load shedding.